Japanese patent publication 2000170538 discloses a two-stroke engine having outlet-near transfer channels which are delimited by set walls relative to the interior space of the cylinder. The set wall must be secured in its position. At the same time, existing tolerances must be compensated.
It is an object of the invention to provide a two-stroke engine which is advantageously manufactured and to also provide a method for making the two-stroke engine.
The two-stroke engine of the invention includes a two-stroke engine for a portable handheld work apparatus and the two-stroke engine includes: a crankcase; a cylinder connected to the crankcase; the cylinder having a cylinder wall defining an interior space; a piston displaceably mounted in the cylinder and the piston and the cylinder conjointly defining a combustion chamber; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft so as to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder between bottom dead center and top dead center; the cylinder having a discharge outlet formed therein for conducting exhaust gases away from the combustion chamber; an inlet leading into the crankcase; at least one transfer channel for fluidly connecting the crankcase to the combustion chamber at pregiven positions of the piston and the transfer channel opening into the cylinder via an inlet window; a set wall for delimiting the transfer channel with respect to the interior space of the cylinder over at least a portion of the longitudinal extent of the transfer channel; and, holding means for holding the set wall with respect to the interior space with the holding means being formed as one piece with the cylinder.
The number of individual parts to be assembled is reduced by securing the position of the set wall relative to the interior space of the cylinder by means which are configured as one piece with the cylinder. The means are produced with the cylinder in one method step. No additional holding means need be provided in the direction of the interior space of the cylinder. For this reason, the interior wall of the cylinder is unaffected by the holding means and therefore the piston running path is also unaffected thereby.
The set wall is held with respect to the cylinder interior space at at least one longitudinal end running approximately perpendicular to the peripheral direction of the cylinder. This is especially practical when the ends, which lie in the direction of the cylinder longitudinal axis, simultaneously define boundaries of the inlet window in the combustion chamber and the outlet window in the crankcase. Furthermore, this configuration makes possible the axial insertion of the set walls during assembly approximately in the direction of the longitudinal axis of the cylinder. The set wall is advantageously mounted in an assembly opening in the cylinder. The width of the assembly opening is measured perpendicularly to the longitudinal axis of the cylinder and is especially greater than the width of the transfer channel arranged in the region of the assembly opening. The set wall thereby projects beyond the transfer channel in the peripheral direction. In this way, a reliable sealing of the transfer channel to the interior space of the cylinder is ensured. At the same time, structural space for further means for fixing the set wall is provided, especially in the radial direction toward the outside.
The means for fixing the set wall in the direction toward the interior space of the cylinder are advantageously formed in that the distance of the longitudinal ends of the assembly opening is less on the end, which faces the interior space of the cylinder, than the width of the set wall at the same elevation. The width of the set wall is measured approximately in the peripheral direction and the longitudinal ends of the assembly opening run approximately in the direction of the longitudinal axis of the cylinder. With the above configuration, the set wall comes to rest in the region of its longitudinal ends behind the assembly opening viewed toward the interior space of the cylinder. To simplify assembly and to compensate for tolerances, it is provided that the set wall and the assembly opening are inclined relative to the longitudinal axis of the cylinder at their longitudinal ends. The opposite-lying longitudinal ends of the set wall and the opposite-lying longitudinal ends of the assembly opening run toward each other in the direction toward the combustion chamber.
To avoid affecting the running path of the piston, it is provided that the interior wall surface of the set wall, which faces the interior space of the cylinder, is offset relative to the cylinder interior wall radially toward the outside. A contact of the piston with the set wall is avoided in this manner. Advantageously, several, and especially two transfer channels, are delimited by a common set wall. In this way, the number of parts can be further reduced. At the same time, the assembly of this larger set wall is facilitated. Taken together, all set walls extend over 35% to 70% of the cylinder interior wall.
The set wall is fixed by a holding element to compensate for tolerances. The holding element especially fixes the set wall in the radial direction toward the outside. The holding element has spring-like or resilient characteristics at least in the radial direction to the longitudinal axis of the cylinder for compensating for tolerances. The holding element is arranged on the end of the set wall facing away from the interior space of the cylinder and is especially accommodated in a receptacle between the set wall and the cylinder. The holding element can press the set wall in this way in a direction toward the interior space of the cylinder against the means which hold the set wall in the direction toward the interior space of the cylinder. In this way, a reliable fixing of the set wall is ensured. The holding element is especially provided as a clamp pin. However, it can also be practical that the holding element be an adhesive. The holding element advantageously extends over a large portion of the length of the set wall. A simple manufacture and a reliable fixing can be achieved when the set wall is fixed by a weld or a solder. It can, however, also be advantageous that the set wall is pressed into the cylinder.
For the method of making a two-stroke engine with transfer channels open to the interior space of the cylinder, it is provided to manufacture the cylinder in the pressure die cast method. At least one set wall is pushed into the cylinder between the interior space of the cylinder and the transfer channel and the set wall is fixed after being pushed in by a holding element. The insertion of the set wall and the fixing thereof via a holding element makes possible a simple and cost effective manufacture. With the holding element, manufacturing tolerances can be compensated in a simple manner. At the same time, the set wall is reliably held in the cylinder.
The holding element is especially pushed in approximately parallel to the insert direction of the set wall and especially approximately in the direction of the longitudinal axis of the cylinder from the crankcase in the direction of the combustion chamber. The set wall is pushed into the cylinder after the cylinder is coated.
For a good fixing of position, the fixation of the set wall can be advantageously done with adhesive. It is also practical to fix the set wall with welding or soldering. In this way, additional attachment components are not necessary. Additional components are also avoided when the set wall is pressed in.